1. Field of the Invention
This invention relates to improved process aid compositions for the melt extrusion of difficultly-melt-processible polymers.
2. Background
In the melt extrusion of polymer resins there are often flow regimes, determined by the rheological properties of the particular resin, where anomalous flow behavior occurs leading to surface imperfections on the extrudate surfaces. Such imperfections, commonly called melt fracture, appear in different forms. The so-called "sharkskin" fracture occurs at lower shear rates and appears as a general, finely-structured and uniform roughness. In a blown-film extrusion sharkskin fracture may appear as an undesirable herringbone pattern, reducing clarity and giving a dull surface. In practice this may occur at uneconomically low extrusion rates. At higher shear rates flow often becomes unstable and a non-uniform stick-slip melt fracture results, wherein alternating bands of glossy surface and sharkskin fracture appear. This behavior is especially undesirable in wire coating and in tube and pipe extrusions as well as in blown-film applications. Other well-known problems that create difficulties in extrusion include fluctuations in barrel and die pressure, torquing out because of the excessively high pressure reached during a fluctuation, and accumulation of degraded polymer at the die exit orifice.
In an effort to improve the extrusion behavior of polymer resins through metal dies it is known to coat the die surfaces that contact the flowing polymer melt with a slip agent, such as tetrafluoroethylene polymers and copolymers, as in Japanese Application Publication Kokai 55-82784 (Mitsui Petrochem. Ind., KK), but bonding to the metal is poor, and over a period of time in use the slip layer is depleted and melt fracture resumes.
In other practices, as for example in the extrusion of certain hydrocarbon polymers and copolymers, it is known to employ small amounts of fluorocarbon polymers, blended with the extrusion resin, as a continuously replenishing slip agent. Thus Blatz, in U.S. Pat. No. 3,125,547, discloses hydrocarbon polymer compositions having improved extrusion behavior that contain small amounts of fluorocarbon polymers that are above their glass transition temperature, if amorphous, or above their crystalline melting point (e.g. molten), if crystalline, at the process temperatures. Under these conditions the flow rate above which melt fracture occurs is greatly increased, and required extrusion pressures for a given extrusion rate are diminished. Takeshi and Inui in Japanese Examined Application Kokoku 70-30574 disclose continuous extrusion molding of polyethylene compositions containing small amounts of tetrafluoroethylene polymer (crystalline at process temperatures). Japanese Unexamined Application Kokai 1,074,247 describes the use of certain combinations of fluoropolymer process aids disclosed in U.S. Pat. No. 3,125,547, cited above. U.S. Pat. No. 4,904,735 discloses the use of combinations of fluoropolymers that are molten at process temperatures, such as fluoroelastomers, and those that are not molten at process temperatures, such as crystalline tetrafluoroethylene homopolymers and copolymers.
Japanese Examined Applications Kokoku 55543/1988 and 55544/1988 describe compositions comprising a thermoplastic resin and a fluoropolymer process aid having pendant --SO.sub.3 M groups, where M is an alkali metal anion.
The important effect of polar functionality situated on the fluoropolymer chain has not been heretofore recognized. It is an objective of this invention to describe fluoropolymer compositions having effective concentrations of polar functionality and enhanced utility as process aids for the extrusion of difficultly-melt-processible polymers.